In an air conditioner, one or a plurality of outdoor units (heat-source side units), each having a compressor and an outdoor heat exchanger (heat-source side heat exchanger), and one or a plurality of indoor units (load-side units), each having a throttle device so as to become an expansion valve and an indoor heat exchanger (load-side heat exchanger), are connected by a pipeline. A space to be air-conditioned is cooled/heated by configuring a refrigerant circuit so as to circulate a refrigerant.
When the outdoor unit is performing a heating operation, for example, a low-temperature refrigerant passes through a pipeline in the outdoor heat exchanger which becomes an evaporator, and heat exchange is performed between the refrigerant and air through the pipeline, and thus, moisture in the air is condensed in a fin or in a heat transfer pipe and forms frost. If the frost accumulates (frost formation), the heat exchange with air cannot performed well, and a heating capacity (a heat amount per time to be supplied to the indoor unit side (hereinafter, this capacity also including a cooling capacity is referred to as capacity)) in the outdoor unit deteriorates, and the capacity cannot be exerted for an air-conditioning load (a heat amount required by the indoor unit (hereinafter, referred to as a load)) in the indoor unit. Then, in order to remove the frost formed on the heat-source side heat exchanger during heating, for example, a defrosting operation (defrosting) is performed for each outdoor unit (See Patent Document 1, for example). At this time, the defrosting operation is performed in any one of the outdoor units, while the heating operation is continued in the other outdoor units.
For example, in the outdoor unit that performs the defrosting operation, a four-way valve is switched so that a hot gas (a high-temperature gas refrigerant) from the compressor directly flows into the outdoor heat exchanger. Through heat exchange between the hot gas and the frost, the frost is melted, and the hot gas is partially liquefied and brought into a gas-liquid two-phase refrigerant. This gas-liquid two-phase refrigerant and the high-temperature gas refrigerant coming out of the outdoor unit that continues the heating operation are combined, the high-temperature two-phase refrigerant flows to the indoor unit side, and cooling/heating is performed.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2007-271094